1. Field of the Invention
The present invention relates to a twin-wire former for producing a fibrous material web, e.g., a paper or cardboard web, from a fibrous suspension. The invention may also be related to further drainage of the fibrous material web. The present invention is based on a twin-wire former that includes two endless wire belts, e.g., a lower wire and upper wire, which jointly form a twin-wire zone. The upper wire is guided by at least two rolls, e.g., an inlet roll, which guides the upper wire to the twin-wire zone, and an outlet roll, which guides the upper wire back toward the inlet roll, and the upper and lower wires form a wedge-shaped inlet zone having an adjustable wedge angle. An upper wire subassembly, which includes a drainage box to receive suspension water flowing upwardly through the upper wire, as well as a common support device for the drainage box and for the inlet roll, is provided. The common support device can be pivoted via a stroke device around a horizontal "operational pivot" axle to vary the wedge angle during former operation.
2. Discussion of Background Information
Systems similar in general to the above-noted device have been discussed, e.g., in German Application No. 40 05 420, of which U.S. Pat. No. 5,045,153 is a patent family member; German Patent No. 43 26 867, of which U.S. Pat. No. 5,554,267 is a patent family member; and European Application No. 0 397 430, of which U.S. Pat. No. 5,074,964 is a patent family member.
The above noted documents, in contrast to the present invention, relate exclusively to so-called hybrid formers, in which a fibrous suspension flows onto the lower wire first. The web formation begins on the lower wire by removing water downwardly through the lower wire. Then the upper wire comes into contact with the remaining fibrous suspension in the twin-wire zone so that at this point, further web formation occurs via removing water upwardly through the upper wire, and, generally, downwardly through the lower wire. As a rule, the web formation is completed in the region of the twin-wire zone, i.e., there are no more free floating fibers, and, more often than not, from this point on, further drainage of the fibrous material web occurs to increase the dry matter content as much as possible.
In the twin-wire zone, it is possible, if necessary, to vary the inlet wedge angle (i.e., the angle of the wedge-shaped inlet zone) during operation so as to adapt the twin-wire former to various layer thicknesses of the incoming fibrous suspension. Sometimes, it is also desirable to move the point at which the upper wire comes into contact with the fibrous suspension, i.e., either in the web travel direction or in the opposite direction. To this end, it is known to pivot the upper wire subassembly with a stroke device around an operational pivot axle. In the known twin-wire formers, at least one additional (and costly) stroke device is provided to vertically adjust the position of the operational pivot axle. This adjustment via the additional stroke device is used for the purpose of lifting the upper wire subassembly up from the lower wire, e.g., to facilitate the changing of the wire and/or to prevent damage to the wires when there is an undesired stoppage of one of the two wires.
In the above-cited references, a number of different positions are provided for the operational pivot axle. According to a variant shown in FIG. 2 of European Application No. 0 397 430, the operational pivot axle is located in the vicinity of the point at which the wires emerge from the drainage box (which is positioned inside the loop of the upper wire). This arrangement, which may be utilized in the twin-wire former according to the present invention, is favorable because, with a change of the inlet wedge angle, there is no horizontal motion of any consequence for the strips of the drainage box (which touch the upper wire) relative to the drainage elements located inside the lower wire loop. Such relative motion would impair the proper formation of the fibrous web. Also, the outlet end of the drainage box is prevented from vertical motion of any consequence, which would impair the proper travel of the two wires to the subsequent wire-separating device.